Electro-magnetic actuator for torque coupling with variable pressure-relief valve

ABSTRACT

A torque-coupling assembly comprises a rotatable torque-coupling case, a friction clutch assembly for selectively engaging the torque-coupling case and an output shaft, a hydraulic pump for generating a hydraulic fluid pressure and a variable pressure-relief valve assembly to selectively control the clutch assembly. The variable pressure-relief valve assembly includes a valve closure member, a valve seat and an electro-magnetic actuator for engaging the valve closure member and generating a variable axial electro-magnetic force acting to the valve closure member so as to selectively vary a release pressure of the pressure-relief valve assembly. The electro-magnetic actuator includes an inverted electro-magnetic coil assembly and an armature disposed inside the electro-magnetic coil assembly and axially movable relative thereto. The torque-coupling assembly further comprises at least one plenum passage provided adjacent to an inner peripheral surface of the armature for directing hydraulic fluid exiting the torque-coupling case through the electro-magnetic actuator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to torque coupling assemblies in general,and more particularly to a torque coupling assembly including anelectro-magnetic actuator controlling a variable pressure-relief valve.

2. Description of the Prior Art

Hydraulic couplings are used in various vehicular drivetrainapplications to limit slip and transfer drive torque between a pair ofrotary members. In all-wheel drive applications, hydraulic couplings areused to automatically control the drive torque transferred from a drivenmember to a non-driven member in response to speed differentiationtherebetween. In limited slip applications, couplings are used inassociation with a differential to automatically limit slip and bias thetorque distribution between a pair of rotary members.

Such hydraulic couplings conventionally use a frictional clutch betweenthe rotary members. The frictional clutch may be selectively actuated byvarious hydraulic actuator assemblies, which are constructed of elementsdisposed inside the differential case. The hydraulic actuator assembliesinternal to a torque-coupling case often include displacement pumpsdisposed inside the torque-coupling case and actuated in response to arelative rotation between the torque-coupling case and the output shaft.The displacement pumps are usually in the form of internal gear pumps,such as gerotor pumps adapted to convert rotational work to hydraulicwork. In the internal gear pumps, an inner gear having outwardlydirected teeth cooperates with an external gear having inwardly directedteeth so that fluid chambers therebetween increase and decrease involume as the inner and outer gears rotate in a housing.

Pump type hydraulic couplings, such as active limited slipdifferentials, employ the internal pump to convert the spin speeddifference between the one of the output shafts and the differentialcase to a hydraulic pressure that actuates a piston actuator (hydrauliccylinder), which in turn activates a multi-plate clutch pack. Inaddition, an electromagnet-activated pressure-relief valve, disposed ata fluid outlet hole of the pump, controls the fluid pressure and thusthe torque level of the limited slip action. Prior-art pump type activelimited slip differentials are acceptable for low-speed mobilitysituations (e.g. split-μ hill climb), but they lose its controllabilityfor medium-to-high-speed handling maneuvers. This failure of thepump-type coupling is caused by the fact that fluid inlet and outletholes positioned at significantly high outer radius of the differentialcase, resulting in the centrifugal loss/draining of the fluid from thepump and the piston when the differential case is spinning.

The reason for this is that the present pump-type active limited slipdifferentials employ an annular electromagnet that is oriented upright,i.e. open at its outer radius. An annular armature is disposed at theouter radius of the electromagnet with a small amount of axial positionoffset. The energized electromagnet axially pulls the armature towardsthe differential case, choking the pressure-relief valve disposed at theoutlet hole of the differential case. Such a radial arrangement of theelectromagnet and armature in the prior art pump type active limitedslip differentials renders no choice but to position the fluid inlet andoutlet holes at the radial position equal to or larger than the radiusof the armature, which is usually larger than the diameter of thehydraulic pump and the piston. As a result, when the differential casespins in response to the vehicle speed, the hydraulic fluid in the pumpand a piston chamber of the piston actuator is centrifugally drainedthrough the oil inlet and outlet holes, resulting in the failure of thedifferential system in terms of time delay and abrupt engagement of theclutch. Therefore, the prior-art pump type active limited slipdifferentials fail to work for medium-to-high-speed handling maneuvers.

Thus, while known hydraulic couplings, including but not limited tothose discussed above, have proven to be acceptable for some vehiculardriveline applications and conditions, such devices are neverthelessunacceptable for some operational conditions and susceptible toimprovements that may enhance their performance and cost. With this inmind, a need exists to develop improved hydraulic torque-couplingassemblies that advance the art.

SUMMARY OF THE INVENTION

The present invention provides an improved electronically controlledtorque-coupling assembly. The torque-coupling assembly in accordancewith the present invention comprises a rotatable torque-coupling case,at least one output shaft drivingly operatively connected to thetorque-coupling case, a friction clutch assembly for selectivelyengaging and disengaging the torque-coupling case and the output shaft,a hydraulic pump for generating a hydraulic fluid pressure tofrictionally load the clutch assembly, and a variable pressure-reliefvalve assembly to selectively control the friction clutch assembly.

The variable pressure-relief valve assembly includes a valve closuremember, a valve seat complementary to the valve closure member, and anelectro-magnetic actuator for engaging the valve closure member andgenerating a variable axial electro-magnetic force acting to the valveclosure member so as to selectively vary a release pressure of thepressure-relief valve assembly based on a magnitude of an electriccurrent supplied to the electro-magnetic actuator. The electro-magneticactuator includes an inverted electro-magnetic coil assembly and anarmature disposed inside the electro-magnetic coil assembly and axiallymovable relative thereto. The torque-coupling assembly further comprisesa plenum passage provided adjacent to an inner peripheral surface of thearmature that allows fluid communication through said electro-magneticactuator.

Therefore, the electronically controlled torque-coupling assembly inaccordance with the present invention is provided with anelectro-magnetic actuator for activating a variable pressure-reliefvalve for allowing continuously variable torque coupling anddistribution. The inverted radial arrangement of the electro-magneticcoil assembly allows the inlet and outlet holes be positioned at asmaller radial location for effectively eliminating the centrifugalfluid loss problem.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent froma study of the following specification when viewed in light of theaccompanying drawings, wherein:

FIG. 1 is a schematic diagram of a drive axle assembly of a motorvehicle including an electronically controlled torque coupling assemblyin accordance with the present invention;

FIG. 2 is a sectional view of the torque coupling assembly according tothe preferred embodiment of the present invention;

FIG. 3 is an enlarged cross-sectional view of a variable pressure-reliefvalve assembly of the present invention mounted to a side cover memberof a differential case of the torque coupling assembly;

FIG. 4 shows structure and method for manufacturing of an invertedelectro-magnetic coil assembly according to the preferred embodiment ofthe present invention;

FIG. 5 is a plan view of a bushing member and an armature in thedirection of arrows A-A in FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiment of the present invention will now be describedwith the reference to accompanying drawings.

For purposes of the following description, certain terminology is usedin the following description for convenience only and is not limiting.The words “right,” “left,” “lower,” and “upper” designate directions inthe drawings to which reference is made. The words “uppermost” and“lowermost” refer to position in a vertical direction relative to ageometric center of the apparatus of the present invention anddesignated parts thereof. The terminology includes the words abovespecifically mentioned, derivatives thereof and words of similar import.Additionally, the word “a,” as used in the claims, means “at least one.”

The present invention is directed to a hydraulically actuated torquecoupling assembly including a hydraulic fluid pump, such as ahydraulically controlled limited slip differential (LSD) assemblyindicated generally at 10 in FIGS. 1 and 2 that illustrate the preferredembodiment of the present invention. It will be appreciated that thehydraulically actuated torque coupling assembly of the present inventionmay be in any appropriate form other than the limited slip differentialassembly, such as hydraulically actuated shaft coupling, auxiliary axlecoupling for a motor vehicle, a power take-off coupling of afront-wheel-drive transaxle, etc.

FIG. 1 schematically depicts a rear wheel drive axle assembly 1including the differential assembly 10. The differential assembly 10comprises a differential case (or a coupling case) 12 rotatablysupported within a differential housing (or a coupling housing) 4 anddriven by a drive pinion gear 3 transmitting a drive torque from anengine (not shown) to a ring gear 14 through a propeller shaft 5. Adifferential gear mechanism 15 disposed within the differential case 12is operatively coupled to output axle shafts 8 a and 8 b for allowingdifferential rotation thereof. As further shown in FIGS. 1 and 2, thedifferential assembly 10 is provided with a hydraulic coupling which isresponsive to differences in rotations between the output axle shafts 8a, 8 b. The hydraulic coupling comprises a friction clutch assembly 20and an actuator assembly operably arranged to actuate the clutchassembly 20 for automatically and progressively transferring drivetorque from the faster rotating axle shaft to the slower rotating axleshaft in response to excessive speed differentiation therebetween. Theactuator assembly includes a hydraulic fluid pump 26, a piston assembly27, and a variable pressure-relief valve assembly 30 for selectivelycontrolling a discharge pressure of the pump 26 and, subsequently, theclutch assembly 20. Preferably, the clutch assembly 20 is ahydraulically actuated multi-plate friction clutch assembly. However,other appropriate types of hydraulically actuated clutches are withinthe scope of the present invention. The hydraulic fluid pump 26 providespressurized hydraulic fluid for actuating the clutch assembly 20. Boththe actuator assembly and the clutch assembly 20 are disposed within thedifferential case 12.

The variable pressure-relief valve assembly 30 is operated by anelectro-magnetic (preferably, solenoid) actuator, electronicallycontrolled by a differential control module (DCM) 6 based on one or morevehicle parameters 2 a as control inputs, such as a vehicle speed, awheel speed difference, vehicle yaw rate, a vehicle lateralacceleration, a steering angle, an engine throttle position, a brakeapplication, an ice detection, a moisture detection, a vehicle drivelineconfiguration and a yaw stability control system actuation, and aprogrammable control mechanism could be used to interface with thehydraulic actuated limited slip differential. The DCM 6 is alsoconnected to a source of an electric power supply, such as an electricstorage battery 2 b mounted on the motor vehicle.

When energized, solenoid-operated valve assembly 30 is capable ofmodulating a pump discharge pressure in a variable range from a minimumpressure to a maximum pressure, thereby variably controlling a drivetorque distribution between the output axle shafts 8 a and 8 b in arange from a minimum torque value to a maximum torque value. Forexample, the yaw stability control actuation may be actuated when thevehicle yaw rate reaches a predetermined level. At this same yaw rate,the variable pressure-relief valve assembly 30 will be actuated todisengage the limited slip feature of the LSD assembly 10. When thevehicle yaw rate falls below the predetermined level, the limited slipfeature can be turned back “ON”. Another vehicle parameter which couldbe effectively used in this manner is the steering angle. This could besensed to determine if the limited slip feature is needed. At rangesfrom small to no steering angle the limited slip feature can be madeavailable and then disengaged at larger steering angles. Either of thesemethods could also be combined with the previously mentioned method ofoptimizing the amount of limited slip available between an “ON” and“OFF” position by monitoring differences in wheel RPM or speed.

FIG. 2 of the drawings illustrates in detail the preferred arrangementof the differential assembly 10 in accordance with the presentinvention. The differential case 12 includes a case member 12 a and aside cover member 12 b each provided with a complementary annularflange. The flanges of the case member 12 a and the side cover member 12b are fastened to each other by any appropriate means known in the art,such as threaded fasteners (not shown), to form a generally cylindricalstructure and define a flange 12′. The differential case 12 alsoincludes hollow receiving hubs (trunnions) 21 on each end thereof. Thehubs 21 define apertures for receiving opposite output shafts 8 a, 8 b.The differential case 12 is rotatably supported in the differentialhousing 4 (shown only schematically in FIG. 1) for rotation about acentral axis 11 through differential bearings (not shown) mounted aboutthe hubs 21. The differential housing 4 forms a differential chambercontaining a supply of a hydraulic lubricant fluid, thus defining ahydraulic fluid reservoir.

The ring gear 14 (shown in FIG. 1) is bolted or other wise secured tothe differential case 12 at the flange 12′. The differential gearmechanism 15 disposed within the differential case 12 includes a set ofpinion gears 16 rotatably supported on a pinion shaft 17 secured to thedifferential case 12. The pinion gears 16 engage a pair of opposite sidegears 18 a and 18 b adapted to rotate about the axis 11. The side gears18 a and 18 b are splined to the output axle shafts 8 a and 8 brespectively. Disposed adjacent the side gear 18 a is an inner clutchsleeve 19 having external splines and drivingly coupled to theassociated axle shaft 8 a.

The friction clutch assembly 20 of the limited slip device is providedwithin the differential case 12. The friction clutch assembly 20, wellknown in the prior art, includes at least one outer friction plate 22 aand at least one inner friction plate 22 b. Typically, the frictionclutch assembly 20 includes sets of alternating outer friction plates 22a and inner friction plates 22 b. Conventionally, an outer circumferenceof the outer friction plates 22 a is provided with projections thatnon-rotatably engages corresponding grooves formed in the differentialcase 12. Similarly, an inner circumference of the inner friction plates22 b is provided with projections that non-rotatably engagecorresponding grooves formed in the clutch sleeve 19, which in turn issplined to the associated axle shaft 8 a. At the same time, both theouter friction plates 22 a and the inner friction plates 22 b areslideable in the axial direction. The clutch plates 22 a frictionallyengage the clutch plates 22 b to form a torque coupling arrangementbetween the differential case 12 and the differential mechanism 15formed by the pinion gears 16 and side gears 18 a, 18 b. Torque istransferred from a ring gear (not shown) to the differential case 12,which drives the differential mechanism 15 through the pinion shaft 17.

When the friction clutch assembly 20 is actuated by the hydraulic clutchactuator assembly, the outer clutch plates 22 a frictionally engage theinner clutch plates 22 b to form a torque coupling between thedifferential case 12 and the output shaft 8 a. As described below, thehydraulic pump 26 actuates the friction clutch assembly 20 depending onthe relative rotation between the differential case 12 and the clutchsleeve 19, i.e. the axle shaft 8 a. More specifically, the speedsensitive fluid pump 26 actuates the piston assembly 27 that compresses(axially loading) the friction clutch assembly 20 to increase thefrictional engagement between the clutch plates 22 a and 22 b.

The speed sensitive hydraulic displacement pump 26 disposed within thedifferential case 12 actuates the clutch assembly 20 when the relativerotation between the output axle shafts 8 a and 8 b occurs. It will beappreciated that a hydraulic pressure generated by the pump 26 issubstantially proportional to a rotational speed difference between theoutput axle shafts 8 a and 8 b. In the preferred embodiment, thehydraulic displacement pump 26 is a speed sensitive, bidirectionalgerotor pump. The gerotor pump 26 includes an outer ring member 26 a, anouter rotor 26 b, and an inner rotor 26 c. The inner rotor 26 cdrivingly coupled to the output axle shaft 8 a, and the outer ringmember 26 a is secured to the differential case 12. The inner rotor 26 chas one less tooth than the outer rotor 26 b and when the inner rotor 26c is driven, it will drive the outer rotor 26 b, which can freely rotatewithin the outer ring member 26 a eccentrically with respect to theinner rotor 26 c, thus providing a series of decreasing and increasingvolume fluid pockets by means of which fluid pressure is created.Therefore, when relative motion takes place between differential case 12and the output axle shaft 8 a, i.e. between the output axle shafts 8 aand 8 b, the gerotor pump 26 generates hydraulic fluid pressure.However, it will be appreciated that any other appropriate type ofhydraulic pump generating the hydraulic pressure in response to therelative rotation between the differential case 12 and the output axleshaft 8 a is within the scope of the present invention.

The piston assembly 27 including a hydraulically actuated piston 27 adisposed within a piston housing 27 b, serves to compress the clutchpack 20 and retard any speed differential between the side gear 18 a andthe differential case 12. This results in a retardation of any speeddifferential between the axle shafts 8 a and 8 b. Pressurized hydraulicfluid to actuate the piston 27 a and engage the clutch pack 20 isprovided by the gerotor pump 26. In such an arrangement, when a speeddifference between the output shafts 8 a, 8 b exists, the hydraulicfluid is drawn into the pump 26. The gerotor pump 26 pumps thepressurized fluid into a piston pressure chamber 27 c defined betweenthe piston 27 a and the piston housing 27 b to actuate the clutch pack20. As the speed difference increases, the pressure increases. Thepressurized fluid in the piston pressure chamber 27 c creates an axialforce upon the piston 27 a for loading the clutch pack 20, which isfurther resisted by the friction plates 22 a and 22 b. The loading ofthe clutch pack 20 allows for a torque transfer distribution between theaxle shafts 8 a and 8 b.

The torque coupling assembly 10 further comprises a non-rotatablehydraulic fluid plenum plate 50. As illustrated in detail in FIG. 3, theplenum plate 50 is rotatably mounted to the side cover member 12 b ofthe differential case 12 so as to form an annular, fluidly sealedhydraulic plenum chamber 51 defined between the plenum plate 50 and thedifferential case 12. More specifically, the plenum plate 50 isstationary relative to the differential housing 4, while thedifferential case 12 is rotatable relative thereto. The plenum plate 50includes a pickup tube 54 for supplying the hydraulic fluid from thehydraulic fluid reservoir 4 to the plenum chamber 51. The pickup tube 54has an inlet end 54 a and an outlet end 54 b. The inlet end 54 a of thepickup tube 54 is provided with an inlet opening 57 submerged in theHydraulic lubricant fluid in the hydraulic fluid reservoir 4. In turn,the outlet end 54 b of the pickup tube 54 is provided with an outletopening 59 fluidly connecting the pickup tube 54 with the plenum chamber51. The plenum plate 50 is substantially circular in configuration andincludes an annular outer flange 60 and an annular inner flange 62defining a central opening therethrough.

Moreover, as illustrated in detail in FIG. 3, the LSD assembly 10according to the preferred embodiment of the present invention isprovided with an annular bushing 72 non-rotatably secured to the innerflange 62 of the plenum plate 50. In turn, the bushing 72 is rotatablymounted to the hub 21 of the differential case 12. The bushing 72 ismade of any appropriate non-magnetic material, such as plastic material.Preferably, the plastic bushing 72 is molded over the inner flange 62 ofthe plenum plate 50. A central bore 74 of the plastic bushing 72 isprovided with an inner seal in the form of at least one O-ring seal 75in sealing contact with the hub 21 of the differential case 12. Theouter flange 60 axially extends from the plenum plate 50 toward thedifferential case 12 and is further provided with an annular outer lipseal 76 adapted to engage an outer peripheral surface of an annularflange 12 c extending axially outwardly from the side cover member 12 bof the differential case 12. Thus, the inner and outer seals 75 and 76,respectively, fluidly seal the plenum chamber 51 between the plenumplate 50 and the side cover member 12 b of the coupling case 12.

As further shown in FIGS. 2 and 3, the casing side cover member 12 b hasat least one, preferably more than one, inlet hole (or port) 24 throughwhich the hydraulic fluid is drawn into the hydraulic fluid pump 26 fromthe plenum chamber 51 (depicted by the reference mark F₁ in FIG. 3), andat least one outlet hole (or port) 36 through which the hydraulic fluidexits the differential case 12 and into the plenum chamber 51 (depictedby the reference mark F₂ in FIG. 3). The outlet hole 36 is in fluidcommunication with the piston pressure chamber 27 c. Preferably, theinlet and outlet holes 24 and 36, respectively, are formed in thedifferential case 12 on radii as close to the central axis 11 aspossible so as to eliminate the centrifugal hydraulic fluid loss problemof the prior-art pump-type torque-coupling systems. Further preferably,the inlet and outlet holes 24 and 36 are formed in the side cover member12 b of the differential case 12 by drilling. Alternatively, the holes24 and 36 could be formed by casting, or any other appropriate methodknown in the art.

In order to control the fluid pressure in the piston pressure chamber 27c and, subsequently, the output torque distribution of the limited slipdifferential assembly, the variable pressure-relief valve assembly 30 isprovided. The variable pressure-relief valve assembly 30 according tothe present invention, illustrated in detail in FIG. 3, is in the formof an electro-magnetic valve assembly and comprises a pressure-reliefvalve 32 controlled by an electro-magnetic actuator 34 that may be anyappropriate electro-magnetic device well known in the art, such assolenoid.

The pressure-relief valve 32 comprises a valve seat 38 that is in opencommunication with the outlet hole 36, and a substantially conical valveclosure member 40 is adapted to seat in the valve seat 38 for sealingthe outlet hole 36. It will be appreciated that the valve closure member40 may be in any appropriate form other than conical, such as spherical.The valve seat 38 is formed at an outward end of the outlet hole 36 inthe cover member 12 b of the differential case 12. The valve closuremember 40 is movable between a closed position when the valve closuremember 40 engages the valve seat 38, and an open position when the valveclosure member 40 is axially spaced from the valve seat 38, thus forminga throttle valve. FIG. 3 shows the pressure-relief valve 32 in thepartially closed position so that the valve closure member 40 partiallyblocks the outlet hole 36 at the valve seat 38.

As best shown in FIG. 3, the electro-magnetic actuator 34 is disposed inthe plenum chamber 51. The electro-magnetic actuator 34 is alsorotatably mounted to the differential case 12 through the plasticbushing 72. The electro-magnetic actuator 34 comprises an inverted,annular electro-magnetic coil (or solenoid) assembly 42 and an annulararmature 52 axially movable in the direction of the central axis 11.

As further shown in detail in FIG. 4, the inverted electro-magnetic coilassembly 42 comprises an annular plastic bobbin 43 having asubstantially U-shaped cross-section, a coil wire 44 wound on the bobbin43 to define a coil winding 45, two generally identical annularhalf-piece coil housing members 46 a and 46 b inclosing the coil winding45 therebetween so as to form a coil housing 46, and an annular lockmember 47 interlocking the two half-piece coil housing members 46 a and46 b so as to form a coil housing 46. Preferably, the bobbin 43 is openat its outer radius. Electrical current is supplied to the coil wire 44through supply wires 41 from any appropriate source, such as anelectrical battery. Preferably, as shown in FIGS. 2 and 3, the supplywires 41 connected to the coil wire 44 are laid in the pickup tube 54and reach the coil wire 44 through the outlet opening 59 of the pickuptube 54. Each of the coil housing members 46 a and 46 b is formed of asingle-piece magnetically permeable material, such as conventionalferromagnetic materials. Alternatively, the coil housing members 46 aand 46 b may be formed of a plurality of laminations of the magneticallypermeable material.

Preferably, as illustrated in detail in FIG. 4, each of the half-piececoil housing members 46 a and 46 b is substantially L-shaped, and has afirst annular surface (64 a, 64 b) extending radially outwardly from thecentral axis 11 and a second annular surface (66 a, 66 b) extendingsubstantially along the central axis 11. As shown in FIGS. 3 and 4, thefirst annular surfaces 64 a and 64 b of half-piece coil housing members46 a and 46 b are axially spaced from each other, while the secondannular surfaces 66 a and 66 b are juxtaposed. Thus, as shown in FIGS. 3and 4, the L-shaped half-piece coil housing members 46 a and 46 b areforming the coil housing 46 defining a cavity receiving the coil wire 44and having an open side 70 facing the central axis 11. Moreover, each ofthe first annular surfaces 64 a and 64 b of half-piece coil housingmembers 46 a and 46 b has an opening (68 a, 68 b) defining a centralopening 73 in the coil housing 46. The central opening 73 is providedfor mounting the electro-magnetic coil assembly 42 about the hub 21 ofthe differential case 12 and for receiving the annular armature 52within the coil housing 46. Further preferably, the coil assembly 42 isnon-rotatably mounted to the plenum plate 50 substantially coaxially tothe central axis 11 by a retainer member 48 fastening the lock member 47to the plenum plate 50. Consequently, the coil assembly 42 isnon-rotatable relative to the differential housing 4, while thedifferential case 12 is rotatable relative to the coil assembly 42. Thisinverted radial arrangement of the electro-magnetic coil assembly 42allows the inlet hole 24 and the outlet hole 36 be positioned at asmaller radial location, than with the upright radial arrangement of theelectro-magnetic coil assembly, for effectively eliminating thecentrifugal hydraulic fluid loss problem.

The inverted electro-magnetic coil assembly 42 is manufactured asfollows. First, the plastic, channel-shaped bobbin 43 is provided. Then,the coil wire 44 is wound around the bobbin 43 by spinning the plasticbobbin 43 to form the coil winding 45. Subsequently, the two half-piececoil housing members 46 a and 46 b are slid axially onto the coilwinding 45 so as to enclose the coil winding 45 therebetween. Finally,the two half-piece coil housing members 46 a and 46 b are securedtogether so as to form the coil housing 46. Preferably, the twohalf-piece coil housing members 46 a and 46 b are interlocked by thelock member 47. More preferably, the lock member 47 is made of a plasticmaterial which over-molded along an outer peripheral surface of the coilhousing 46 for positively locking the two coil housing members 46 a and46 b together. Specifically, the plastic lock member 47 is over-moldedover annular connecting flanges 49 a and 49 b of the coil housingmembers 46 a and 46 b, respectively, thus interlocking the coil housingmembers 46 a and 46 b. However, other materials should be consideredwithin the scope of the invention.

Therefore, the plenum plate 50, the electro-magnetic coil assembly 42,the lip seal 76, and the bushing 72 with the seal 75 form a singlesub-assembly, facilitating the assembly process of the LSD assembly 10.

The armature 52 is disposed radially inwardly of the electro-magneticcoil assembly 42 substantially coaxially thereto. Moreover, the armature52 is radially spaced from the electro-magnetic coil assembly 42, thusdefining an air gap 56. Preferably, the valve closure member 40 issecurely attached to the armature 52 by any appropriate manner known inthe art. Alternatively, the valve closure member 40 may be integrallyformed with the armature 52 as a single-piece part. As further shown inFIGS. 3 and 5, the annular armature 52 is mounted within theelectro-magnetic coil assembly 42 about the bushing 72 which supportsthe armature 52 within the electro-magnetic coil assembly 42 for axialmovement in the direction of the central axis 11. In other words, thebushing 72 is disposed in a bore 53 in the armature 52 for guiding theaxial movement of the armature 52. Preferably, the armature 52 isnon-rotatably mounted to the side cover member 12 b of the differentialcase 12 through, for example, two dowel pins (not shown) mounted on thedifferential case side cover member 12 b engaging two dowel holes (notshown) on the armature 52, forcing the armature 52 rotate together withthe differential case 12.

Furthermore, a plenum passage is provided adjacent to a radially innerperipheral surface 55 of the armature 52 (shown in detail in FIG. 5) forallowing fluid communication through the electro-magnetic actuator 34.More specifically, as further shown in FIG. 5, the bushing 72 isprovided at least one, preferably a set of circumferentially distributedaxial grooves 78 formed at its outer peripheral surface 79 thereof toprovide (or facilitate) flow of the hydraulic fluid between the armature52 and the bushing 72 therethrough. The axial grooves 78 define theplenum passage adjacent to both the inlet hole 24 and the outlet hole 36and allowing for a fluid communication between the plenum chamber 51 andboth the inlet hole 24 and the outlet hole 36. More specifically, theplenum passage 78 provides fluid communication between the outletopening 59 of the pickup tube 54 and the inlet and outlet holes 24 and36, respectively. In other words, as hydraulic fluid exits thedifferential case 12, it flows through the plenum passage 78 of thebushing 72. Alternatively, the axial grooves 78 may be formed in thearmature 52 at a peripheral surface of the bore 53 thereof. Preferably,the axial grooves 78 have a generally semi-circular cross-section. Itwill be appreciated that any other appropriate cross-section of theaxial grooves 78 will be within the scope of the present invention.

It will be appreciated by those skilled in the art that the armature 52may have any appropriate shape in the cross-section. Preferably, asillustrated in the exemplary embodiment of FIG. 3, the armature 52 has agenerally U-shaped cross-section with magnetic poles facing the coilwinding 45, similar to those used in reluctance electric motors.Moreover, the mutual geometric arrangement of the armature 52 and thecoil housing members 46 a and 46 b is such as to maintain asubstantially constant axial force applied upon the valve closure member40 by the electro-magnetic actuator 34 as it moves from its closed toopen position. This is achieved by maintaining a proper axial position“off-set” between the armature 52 and the coil housing members 46 a and46 b (and, consequently, the coil winding 45). The term “off-set” isdetermined here as an amount of misalignment between the armature 52 andthe coil housing members 46 a and 46 b in an axial direction, or adistance between an outward face of one of the coil housing members 46 aand 46 b and an outward face of the armature 52 in the direction of thecentral axis 11, as shown in FIG. 3.

In operation, when the rotational speed difference between the outputaxle shafts 8 a and 8 b occurs, the hydraulic pump 26 is activated todraw the hydraulic fluid from the hydraulic fluid reservoir(differential housing) 3 through the pickup tube 54 into the plenumchamber 51, then from the plenum chamber 51 into the hydraulic pump 26through the plenum passage 78 and the inlet hole 24. On the other hand,when the hydraulic fluid pressure within the piston pressure chamber 27c generated by the hydraulic pump 26 exceeds a predetermined pressurelimit, the pressure-relief valve 32 opens, and the hydraulic fluid exitsthe piston pressure chamber 27 c through the outlet hole 36 into theplenum chamber 51, then through the plenum passage 78 to the outletopening 59 of the pickup tube 54.

As best shown in FIGS. 2 and 3, when electrical current is supplied tothe coil wire 44, a magnetic flux is caused to flow through the armature52. The magnetic flux creates an axial force that axially displaces thearmature 52 relative to the electro-magnetic coil assembly 42. Thearmature 52 selectively urges the valve member 40 upon the valve seat 38with a predetermined axial retaining force that is a function of theelectrical current supplied to the coil wire 44. It will be appreciatedby those skilled in the art that the pressurized hydraulic fluid willnot flow through the pressure-relief valve 32 until the hydraulicpressure generated by the gerotor pump 26 results in a reaction forcelarger than the axial retaining force exerted to the armature 52 by themagnetic flux generated by the coil wire 44, thereby pushing the valveclosure member 40 out of the valve seat 38. Therefore, such anarrangement creates a pressure-relief valve with a release pressure thatis a function of the current supplied to the coil wire 44, and providesthe predetermined pressure limit in the hydraulic system. Thus, thevariable pressure-relief valve assembly 30 selectively sets the releasepressure of the pressure-relief valve 32 based on the magnitude of theelectrical current supplied to the electro-magnetic coil assembly 42and, subsequently, defines the magnitude of the pressure within thepiston pressure chamber 27 c.

When a maximum current is applied to the coil winding 45 of the solenoidactuator 34, the retaining force of the pressure-relief valve 32 is atits maximum, thus a maximum release pressure is provided by thepressure-relief check valve 32. In this configuration, the maximumpressure attainable within the differential case 12 is sufficient tofully actuate the hydraulic clutch assembly 20 which results inproviding the limited slip function in the differential assembly 10, andthe limited slip feature is in the fully “ON” condition.

The pressure limit of the pressure-relief valve 32, i.e. the releasepressure of the pressure-relief valve 32, can be adjusted by controllingthe current applied to the coil wire 44 of the electro-magnetic actuator34. As less current is applied to the coil wire 44, less axial retainingforce is exerted to the relief valve 32, thus the less is the releasepressure provided by the relief valve 32. This results in an adjustmentmechanism for lowering the maximum system pressure attainable within thedifferential case 12.

When a minimum current is applied to the coil wire 44 of the solenoidactuator 34, the retaining force of the pressure-relief valve 32 is atits minimum, thus a minimum release pressure is provided by the reliefvalve 32. In this configuration, the limited slip feature is in thefully “OFF” condition in that the maximum pressure which can be obtainedin the differential case 12 is not high enough to engage the clutchassembly 20, essentially disabling the limited slip feature of thehydraulic LSD assembly 10 without affecting the differential capability.

In between the “ON” and “OFF” conditions of the LSD assembly 10 therelease pressure of the relief valve 32 may be set at any value bymodulating the current applied to the coil wire 44 of the solenoidactuator 34. This provides the LSD assembly 10 with a variable maximumpressure limit in which the amount of the limited slip available to thedifferential assembly 10 can be limited and optimized to match variousvehicle operating conditions. This provides an opportunity todynamically control the hydraulic pressure for traction enhancement. Forexample, if the release pressure is set at a low value, a control systemcan be used to sense wheel speeds or speed differences and allow forincreased hydraulic pressure. The increase in pressure available may bea function of the speed difference. This will result in an optimizedamount of limited slip between the fully “ON” and “OFF” conditions.

Therefore, the electronically controlled torque-coupling assembly inaccordance with the present invention represents a novel arrangement ofthe torque-coupling assembly provided with an electro-magnetic actuatorfor activating a variable pressure-relief valve for allowingcontinuously variable torque coupling and distribution. The invertedradial arrangement of the electro-magnetic coil assembly allows the oilinlet and outlet holes be positioned at a smaller radial location,effectively eliminating the centrifugal fluid loss problem.

The description of the preferred embodiments of the present inventionhas been presented for the purpose of illustration in accordance withthe provisions of the Patent Statutes. It is not intended to beexhaustive or to limit the invention to the precise forms disclosed. Forexample, it is to be understood that while the present invention isdescribed in relation to a speed sensitive limited slip differential,the present invention is equally suitable for use in other hydraulicallyactuated torque couplings, such as torque coupling mechanisms for avehicular drive-train utilizing a speed sensitive limited slip device.Additionally, although FIG. 1 shows a rear-wheel drive embodiment of theinvention, the invention is equally applicable to a front-wheel driveconfiguration of the differential system.

The foregoing description of the preferred embodiments of the presentinvention has been presented for the purpose of illustration inaccordance with the provisions of the Patent Statutes. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obvious modifications or variations are possible in light ofthe above teachings. The embodiments disclosed hereinabove were chosenin order to best illustrate the principles of the present invention andits practical application to thereby enable those of ordinary skill inthe art to best utilize the invention in various embodiments and withvarious modifications as are suited to the particular use contemplated,as long as the principles described herein are followed. Thus, changescan be made in the above-described invention without departing from theintent and scope thereof. It is also intended that the scope of thepresent invention be defined by the claims appended thereto.

1. A torque-coupling assembly comprising: a rotatable torque-coupling case; at least one output shaft drivingly operatively connected to said torque-coupling case; a friction clutch assembly for selectively engaging and disengaging said torque-coupling case and said at least one output shaft; a hydraulic pump for generating a hydraulic fluid pressure to frictionally load said clutch assembly; a variable pressure-relief valve assembly to selectively control said friction clutch assembly, said variable pressure-relief valve assembly including a valve closure member, a valve seat complementary to said valve closure member, and an electro-magnetic actuator for engaging said valve closure member and generating a variable axial electro-magnetic force urging said valve closure member against said valve seat so as to selectively vary a release pressure of said pressure-relief valve assembly based on a magnitude of an electric current supplied to said etectro-magnetic actuator; said electro-magnetic actuator including an eleetro-magnetic coil assembly and an armature disposed radially inwardly of said electro-magnetic coil assembly and axially movable relative thereto; a plenum passage provided adjacent to a radially inner peripheral surface of said armature for providing fluid communication through said electro-magnetic actuator; and an annular bushing disposed within an annular bore of said armature and rotatably supporting said electro-magnetic actuator to said torque-coupling case so that said plenum passage is formed between said armature and said bushing.
 2. The torque-coupling assembly as defined in claim 1, further comprising a non-rotatable hydraulic fluid plenum plate mounted to said torque-coupling case so as to form an annular hydraulic plenum chamber defined between said plenum plate and the torque-coupling case, said plenum chamber housing said electro-magnetic actuator.
 3. The torque-coupling assembly as defined in claim 2, wherein said electro-magnetic actuator is non-rotatably secured to said plenum plate.
 4. The torque-coupling assembly as defined in claim 3, further comprising an annular bushing rotatably supporting said plenum plate to said torque-coupling case, said bushing is disposed within an annular bore of said armature and said plenum passage is defined by a set of circumferentially distributed axial grooves formed in said bushing.
 5. The torque-coupling assembly of claim 1, wherein said bushing is made of a plastic material.
 6. The torque-coupling assembly as defined in claim 1, further including a piston assembly disposed within said torque-coupling case between said pump and said clutch assembly and defining a pressure chamber, wherein said variable pressure relief valve assembly selectively controls a maximum hydraulic pressure attainable within said pressure chamber.
 7. The torque-coupling assembly as defined in claim 6, wherein said variable pressure relief valve assembly selectively controls said maximum pressure attainable within said pressure chamber between a maximum release pressure and a minimum release pressure.
 8. The torque-coupling assembly as defined in claim 7, wherein said minimum release pressure is at a level that prevents actuation of said friction clutch assembly.
 9. The torque-coupling assembly as defined in claim 7, wherein said maximum release pressure is at a level that enables complete actuation of said friction clutch assembly. 